Distinguish between spontaneous and induced mutations. Give some examples of mutagens that cause induced mutations.

Spontaneous mutations are changes in the DNA sequence that occur naturally and randomly during a cell's normal activities, such as DNA replication. On the other hand, induced mutations result from exposure to external factors, such as chemicals or radiation, that can alter the DNA sequence.

Spontaneous mutations can occur during DNA replication if DNA polymerase makes a mistake and incorporates an incorrect nucleotide into the sequence. This can lead to base substitutions, insertions, or deletions in the DNA, which may cause changes in an organism's traits. However, spontaneous mutations are relatively rare, because cells have many error-checking mechanisms to fix these mistakes during replication.

Induced mutations are caused by exposure to external factors called mutagens. Mutagens can cause damage to the DNA by altering the structure of the nucleotides, causing crosslinking between the DNA strands, or causing breaks in the DNA backbone. This can lead to a higher mutation rate than spontaneous mutations. Some induced mutations can be harmful, causing diseases like cancer, while others can have no effect or even be beneficial to an organism.

There are various types of mutagens that can cause induced mutations: 1. Chemical mutagens: Examples include alkylating agents (e.g., ethyl methanesulfonate), which add alkyl groups to DNA bases; base analogs (e.g., 5-bromouracil), which resemble natural DNA bases and can be mistakenly incorporated during replication, causing base-pair mismatches; and intercalating agents (e.g., ethidium bromide), which insert themselves between DNA bases, causing insertions or deletions. 2. Physical mutagens: Examples include ultraviolet (UV) radiation, which can cause the formation of pyrimidine dimers in the DNA, leading to errors during replication; and ionizing radiation (e.g., X-rays and gamma rays), which can cause single and double-strand breaks in the DNA. 3. Biological mutagens: Examples include certain viruses, which can insert their own genetic material into the host organism's DNA, causing insertions or deletions, or altering the nucleotide sequence in the process of integration. In summary, spontaneous mutations occur naturally and randomly during a cell's normal activities, whereas induced mutations are the result of exposure to external factors called mutagens. Examples of mutagens that cause induced mutations include chemical agents, radiation, and biological agents.